added a test for voxel_prf using the sample stimulus, response, and estimate numpy arrays downloaded via installation
small aesthetic edits to the test_voxel_prf method
added two more tests for methods in estimation module
RF: changed the print statement in the voxel_prf method
RF: there was an error in how I generated the coordinate matrices for the visual space.  instead of representing the center of each location, the coordinates represented the edge of each location.
RF: fixed another error in generate_coordinate_matrices.  now they will return the samely centered and symmetric deg_x and deg_y for any sized screens including odd or even pixels
TEST: added tests to utilities and modified some tests in estimation